1. We have determined the effect of calcium deficiency on vitamin D kinetics using our previously developed thermospray methodology. We have determined that the half-life of 25OHD is about 50% shorter under situations of low calcium intake and negative calcium balance. On a 950 mg/day Ca intake, 25OHD half life was 9.5 days, comparable to our earlier results. On a low calcium intake, 200 mg/day, the half life was reduced to 4.9 days. These values are consistent with a faster conversion of 25OHD into the biologically active form of vitamin D that would serve to enhance absorption under conditions of low calcium intakes. 2. We have shown that the monohydrate of the (C6H5) 4As + cation formed under electrospray conditions is in equilibrium with the unhydrated ion. By use of the van't Hoff plot methodology, we have the enthalpy change for the addition of a single water to (C6H5)4As+. The value obtained, - 12.6 kCal/mole, is comparable to the value obtained by other workers for the addition of a single water to the Cs+ ion.